This article deals with the potential benefits of portal imaging as a tool for the dosimetric control of a radiotherapy treatment. In the introduction the advantages of dosimetric control with portal imaging are described. Subsequently, the physical problem is considered between patient exit dose and dose at the level of the portal detector. Measurements with phantoms have shown that this relationship is complex: different algorithms have been developed by groups of researchers and some of them are summarised in this article. Finally, three examples of dosimetric control using portal imaging are presented: in vivo dosimetry on the beam axis, dosimetric profiles in two dimensions and portal dosimetry for intensity modulation.